Nickel alloy plating

ABSTRACT

A nickel-boron coating deposited on a substrate which comprises nickel, boron and a material selected from bismuth, tin, tellurium, selenium, indium and gallium.

This invention relates to a nickel-boron coating deposited on a substrate, a process for depositing a nickel-boron coating on a substrate and a coating solution for depositing such a coating.

Plating processes for decorative or protective purposes are widely used in industry. Nickel-alloy plating is commonly used in engineering coating applications where wear resistance, hardness and/or corrosion protection are required. It may also be used for replacing gold as a conductive via in electrical applications. Nickel-alloy plating may be electroless, which means that it does not require an external source of electrical power and may be applied on numerous substrates.

When a nickel alloy is deposited by an electroless plating method, there are a combination of requirements and constraints for the electroless nickel plating process and the nickel coating solution used. The deposited coating is greatly dependent on the coating solution used and notably the choices of:

-   -   the reducing agent;     -   the complexing agent;     -   the stabilizer;     -   any other elements in the coating solution, such as a         surfactant;     -   the pH of the coating solution; and/or     -   the temperature of the coating solution during deposition.

Electroless plating methods are widely known for depositing nickel-phosphorus coatings and nickel-boron coatings. Such methods for nickel-phosphorus coatings and nickel-boron coatings are widely different to each other in practice; the coating baths and coating conditions are incomparable and nickel-boron plating methods are generally more complex than nickel-phosphorus plating methods. As the working conditions are very different between these two methods, and are generally incompatible with each other, it is not feasible to transpose or to adapt knowledge from one method to the other and/or to predict possible results of using an element for one set of conditions in different and incompatible conditions. For example, the nickel-phosphorus plating method is usually carried out at a highly acidic pH, i.e. a pH of less than 6, while the nickel-boron method usually requires a highly alkaline coating solution. Even process control techniques, for example conductivity measurement in nickel-phosphorous baths, are not transferable and may prevent deposition from a nickel-boron bath.

In electroless nickel-boron plating it is common to use, as a stabilizer, a source of heavy metal ions, such as thallium or lead from lead chloride, lead nitrate, lead tungstate or lead acetate. However, these heavy metals are less than ideal in terms of toxicity and potential hazards to the environment and to health. Consequently, use of such metals requires significant precautions before, during and after their use in a coating solution. Furthermore, the deposited coating may comprise traces of the stabilizer, which means that traces of lead or thallium may be present in the nickel-boron coating. If the electroless plating is not processed correctly, for example if the stabilizer is not complexed, the amount of heavy metals in the deposited nickel-boron coating may be higher than desired or acceptable.

One aim of the present invention is to provide an improved coating solution for depositing a nickel-boron coating on a substrate, notably free of undesired heavy metals. A further aim is to provide an improved nickel-boron coating. In accordance with one of its aspects, the present invention provides a method of depositing a nickel-boron coating as defined in claim 1. Additional aspects of the invention are defined in independent claims. The dependent claims define preferred and/or alternative embodiments.

In accordance with other aspects, the present invention provides:

a) a method of depositing a nickel-boron coating on a substrate comprising contacting the substrate with a coating solution comprising:

-   -   a source of nickel ions;     -   a reducing agent comprising a source of boron ions, notably a         source comprising a borohydride;     -   a complexing agent, notably comprising ethylene diamine;     -   a stabilizer agent comprises a source of ions selected from         bismuth ions, tin ions, tellurium ions, selenium ions, indium         ions and gallium ions and combinations of two or more thereof ;         and         b) a nickel-boron coating deposited on a substrate, notably         obtained by the method above, wherein the nickel-boron coating         comprising (in % wt):     -   at least 85% wt of nickel;     -   boron in the range 1.0 to 10% wt; and     -   a material selected from bismuth, tin, tellurium, selenium,         indium, gallium and combinations of two or more thereof, in the         range 0.1-5% wt; and         c) an electroless nickel-boron coating solution for depositing a         nickel-boron coating comprising:     -   a source of nickel ions;     -   a reducing agent comprising a source of boron ions, notably a         source comprising a borohydride;     -   a complexing agent, notably comprising ethylene diamine; and     -   a stabilizer agent comprising a source of ions selected from         bismuth ions, tin ions, tellurium ions, selenium ions, indium         ions, gallium ions and combinations of two or more thereof.

In accordance with further aspects, the present invention provides:

a) a method of depositing a nickel-boron coating on a substrate comprising contacting the substrate with a coating solution consisting essentially of:

-   -   a source of nickel ions;     -   a reducing agent comprising a source of boron ions, notably a         source comprising a borohydride;     -   a complexing agent, notably comprising ethylene diamine;     -   a stabilizer agent comprises a source of ions selected from         bismuth ions, tin ions, tellurium ions, selenium ions, indium         ions, gallium ions and combinations of two or more thereof; and         b) a nickel-boron coating deposited on a substrate, notably         obtained by the method above, wherein the nickel-boron coating         consists essentially of (in % wt):     -   at least 85% wt of nickel;     -   boron in the range 1.0 to 10% wt; and     -   a material selected from bismuth, tin, tellurium, selenium,         indium, gallium and combinations of two or more thereof, in the         range 0.1-5% wt.         The term “consisting essentially of” is intended to limit a         definition or the scope of a claim to the specified materials or         steps and those that do not materially affect the basic and         novel characteristic(s) of the claimed invention.

The source of nickel ions may be selected from nickel salts, nickel sulphate, nickel methane sulfonate and/or nickel chloride. The concentration of nickel ions in the coating solution may be at least 10 g/l, at least 15 g/l or at least 20 g/l and/or less than 50g/l, less than 40 g/l or less than 30 g/l. The concentration of nickel ions in the coating solution may influence the concentration of nickel in the deposited coating. The deposited coating may have a concentration of nickel of at least 75% wt, at least 85% wt or at least 90% wt.

The source of boron ions may be a source comprising or consisting essentially of a borohydride, such as sodium borohydride and/or potassium borohydride. The source of boron ions may comprise boranes, such as an amine borane. The amine borane may be dimethylamine borane (DMAB), monomethylamineborane, trimethylamineborane and/or diethyl amine-borane (DEAB). The concentration of the reducing agent in the coating solution may be at least 0.20 g/l, at least 0.25 g/l, at least 0.35 g/l or at least 0.45 g/l and/or less than 1.20 g/l, less than 1 g/l, less than 0.90 g/l or less than 0.80 g/l. The concentration of boron ions in the coating solution may influence the concentration of boron in the deposited coating. The deposited coating may have a concentration of boron of at least 0.5% wt, at least 1.0% wt or at least 2% wt and/or less than 12% wt, less than 10% wt, less than 7% wt or less than 5% wt.

The complexing agent may comprise an agent selected from the group consisting of: ethylene diamine, ammonium citrate, sodium citrate, potassium acetate, sodium succinate, sodium malonate, citric acid, maleic acid, sodium acetate, ammonia, boric acid, sodium pyrophosphate or combination thereof. The concentration of the complexing agent in the coating solution, notably a complexing agent comprising or consisting essentially of ethylene diamine, may be at least 40 g/l, at least 50 g/l and/or less than 80 g/l, less than 70 g/l in the coating solution. Ethylene diamine is preferable as it is easy to use and stable over a wide range of useful temperatures, notably at high temperature, for example at temperature above 80° C. Furthermore, ethylene diamine is easy to use and stable at alkaline pH, for example at pH greater than 11.

The source of bismuth ions may comprise or consist essentially of bismuth nitrate and/or bismuth tungstate. The source of tin ions may comprise or consist essentially one or more tin halides, for example tin chloride, notably tin (II) chloride SnCl₂.

Materials selected from bismuth, tin, tellurium, selenium, indium and gallium are considered preferable to lead or thallium in respect of environmental and health safety issues. It has been found surprisingly that it is possible to replace lead or thallium by one or more of these materials in a nickel-boron electroless plating whilst maintaining bath and coating conditions suitable for widespread commercial application. Notably, it has been found surprisingly that use of bismuth and/or tin provide a coating having similar mechanical resistance and/or hardness to a coating deposited using lead. Use of tin also improves corrosion resistance of the coating, particularly when submitted to a salt fog test of at least 50 h in accordance to ASTM B117. In one preferred embodiment, the stabilizing agent comprises a source of tin ions tin. IN another preferred embodiment, a nickel-boron coating deposited on a substrate comprises (in % wt): at least 85% wt of nickel; boron in the range 1.0 to 10% wt; and a material selected from bismuth, tin, tellurium, selenium, indium, gallium and combinations of two or more thereof, in the range 0.1-5% wt, wherein the material comprises tin. Furthermore, the coating solution comprising a source of ions of one or more of these materials has been found stable for electroless plating during periods of time of more than 10 minutes, or more than 40 minutes, or more than 100 minutes, or more than 200 minutes. Preferably, a single material selected from bismuth, tin, tellurium, selenium, indium and gallium is used, notably such that the nickel-boron coating comprises ≥0.1 wt %, ≥0.5 wt %, ≥1.0 wt % or ≥2 wt % and/or ≤5 wt % or ≤4 wt % of one of these materials. The material may be a metal. One preferred metal is bismuth; another preferred metal is tin. Alternatively, a combination of two or more of these materials may be used, notably such that the nickel-boron coating comprises ≥0.1 wt %, ≥0.5 wt %, ≥1.0 wt % or ≥2 wt % and/or ≤5 wt % or ≤4 wt % of the combination of these materials. In a preferred embodiment, the single material or combination of two or more of the materials is used in the range 0.1-1.0 wt %.

The coating solution may comprise a surfactant, for example a surfactant selected from the group consisting of Tween 80, Pluronic F-127, polyethylene glycol, tergitol, tergitol 15-S-7, tergitol NP-9, ethylene diamine tetrakis. The use of a surfactant may provide additional properties to the deposited nickel-boron coating, notably the exposed surface of the deposited coating, such as improved brightness.

In a preferred embodiment of the invention, the coating solution is free of undesired heavy metals, notably free of lead and thallium. This also avoids the presence of such undesired metal(s) in the composition of the nickel-boron coating.

The coating solution may also be free of phosphorus. The absence of phosphorus in the coating solution provides a better adherence of the nickel-boron coating on the substrate surface. Furthermore, a coating solution free of phosphorus provides a nickel-alloy coating for applications where an absence of phosphorus is desired. Such applications include electronic devices with or without magnetic properties, for example hard drives.

The coating solution may have a pH of at least 11, preferably at least 13, more preferably in the range 13 to 15, notably during the depositing process. The coating solution may be unstable and not suitable for electroless plating at a pH of less than 11. A pH of at least 11 is preferable when using sodium borohydride as source of boron ions as it is not stable at neutral or acidic pH due to its strong tendency to hydrolyse; at a pH of 11, its half-life of 10 hours is more adapted to industrial processes.

Notably during the deposition of the nickel-boron coating, the coating solution may have a temperature of at least 80° C., preferably at least 90° C., more preferably at least 95° C. This provides an improved deposition rate, notably for a coating solution comprising ethylene diamine. The temperature may be less than 120 ° C. or less than 100° C.

The coating solution according to another aspect of the invention provides a nickel-boron coating which may be harder, more resistant to wear and/or thicker than known nickel-phosphorus coatings and may have better adhesion and/or better brightness than nickel-phosphorus coatings.

The deposition rate of the nickel-boron coating may be at least 3 μm/h or at least 5 μm/h. The deposition rate may be at least 10 μm/h, at least 15 μm/h or at least 20 μm/h. The deposition rate may be selected to improve the homogeneity of the deposited coating. For example, if the deposition rate is ≤8 μm/h, the coating may be more homogeneous and thinner than a coating deposited at a speed of ≥10 μm/h.

The coating solution may be replenished which means that the chemistry of the solution is brought back to its initial conditions, for example concentrations, pH and/or temperature, in order to allow the deposition to continue. For example, the oxidation of the borohydride may be accompanied by a diminution of the pH of the coating solution and thus, it may be desired to compensate by addition of a strong base.

The nickel-boron coating may have a thickness of ≥2 μm, ≥5 μm, or ≥10 μm and/or ≤100 μm, ≤75 μm, ≤50 μm, ≤30 μm, 5 20μm or ≤15 μm, notably without a replenishment step.

The nickel-boron coating is preferably subjected to a heat treatment, notably subsequent to its formation. The energy brought to the coating during deposition may not be sufficient to allow formation of a desired equilibrium structure. Furthermore, the coating may be not fully crystallised and thus may present an amorphous or nanostructured character. The heat treatment may also increase the hardness of the coating. The heat treatment may last at least 30 minutes, at least 60 minutes, at least 120 minutes, at least 240 minutes or at least 300 minutes, with a temperature of at least the crystallisation temperature of the coating. The heat treatment may be carried out at a temperature of at least 280° C. or at least 300° C. and/or less than 500° C. or less than 450° C. Preferably, the temperature of the heat treatment is in the range 300° C. to 450° C. If the temperature during the heat treatment is too high, it may lead to softening due to excessive grain growth whilst low temperature does not guarantee full crystallisation.

The substrate may be a conductive substrate or a non-conductive substrate. The substrate may comprise steel, for example carbon steel or mild steel, for example St37.

The nickel-boron coating may comprise or consist essentially of a chemical composition of:

-   -   at least 85% wt, preferably at least 90% wt of nickel; and/or     -   between 1.0 to 10% wt of boron, preferably between 5 and 7% wt;         and/or     -   between 0.1 and 5% wt of a material selected from bismuth, tin,         tellurium, selenium, indium, gallium and combinations of two or         more thereof, preferably between 1.0 and 5% wt, more preferably         between 2 and 4% wt.

In a preferred embodiment, the amount of boron in the nickel-boron coating is between 5 and 7% wt. This corresponds to an atomic amount of about 25% and in this case the nickel-boron forms the crystal structure of Ni₃B, i.e. an orthorhombic crystal structure. The coating may have a hardness, notably a cross section hardness, greater than hard chrome.

The nickel-boron coating may have one or more of the following physical properties:

-   -   a roughness Ra of ≤1 μm, preferably ≤0.7 μm, more preferably         ≤0.3 μm;     -   a roughness Rp of ≤1.5 μm, preferably, ≤1 μm, more preferably         ≤0.7 μm;     -   a friction coefficient of ≤0.5, preferably ≤0.4, more preferably         ≤0.3;     -   a critical load for first damage of at least 15 N, preferably at         least 20 N, more preferably at least 30 N;     -   a cross section hardness of at least 700 hv₁₀₀, preferably at         least 800 hv₁₀₀, more preferably at least 900;     -   a specific wear rate of ≤4 μm²/N, preferably ≤3 μm²/N, more         preferably ≤2 μm²/N;     -   a wear tracks width of ≤600 μm, preferably less than 500 μm,         more preferably ≤400 μm and even more preferably ≤300 μm.

The friction coefficient may be measured following the standard ASTM G99 (as in force on 1 Jan. 2018) the contents of which are incorporated herein by reference.

Tribological behaviour of the samples may be measured with a pin-on-disk CSM microtribometer. The coated samples serve as the disks and the counterparts are 6 mm diameter alumina balls with hardness of 1400 hv₁₀₀, and the sliding speed and sliding distance are respectively 100 cm/s and 100 m. The wear tests are preferably carried out under loads of 10 N. The wear track width is the average value obtained after ten tests.

The specific wear rate (Ws) may be calculated following the European standard EN 1017-13:2008, the contents of which are incorporated herein by reference.

The hardness may be measured on the surface of the substrate comprising the coating with a hardness tester under a load of 100 g and on polished cross sections with a load of 20 g.

The scratch test is one of the methods which allow assessing adherence, fracture and/or deformation of the coating on the substrate. It provides information of the behaviour of the coating/substrate system under a load useful in evaluating its performance in real applications. The scratch test which thus allows determining the critical load for first damage is preferably performed on a steel substrate with a coating having a thickness of 20 μm. The load is applied by a diamond stylus with a conical tip (Rockwell C type geometry cone with 120° and 200 μm spherical radius) at a speed of 100N/min, a transverse velocity of the indenter of about 6.75 mm/min until a maximal load of 150 N. The length of the scratch obtained is of about 10 mm. Preferably, the critical load of first damage is the average value obtained after three consecutive measurements.

The roughness of the surface may be measured by moving a stylus placed on the sample surface, at very low force so to prevent the sample scratching. The vertical movement of the stylus is recorded by a magnetic system. The stylus cannot detect valleys if valleys radius is lower than the stylus radio. An apparatus suitable for such measurement may be Zeiss 119 Surfcom 1400D-3DF. The measurements are measured, for example, with the following parameters:

-   -   length measurement: 12 mm;     -   measuring speed: 0.3 mm/s;     -   cutting wavelength: 0.8 mm;     -   expansion of measurement: 100.         The roughness is determined as the average value of 10         consecutive measurements.

Coating and/or coating solutions in accordance with the present invention are preferably compliant and/or facilitate compliance with ELV 2000/53/EC and/or RoHS 2002/95/EC, the contents of which are incorporated herein by reference.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawing of which:

FIG. 1 illustrates samples of coatings submitted to a neutral salt fog test.

An aqueous alkaline solution (pH 13.5-14.2) for the preparation of a coating solution is prepared comprising:

-   -   about 24 g/L nickel chloride as a source of nickel ions;     -   about 60 g/L ethylene diamine as a complexing agent;     -   about 0.02 g/L of bismuth tungstate oxide as a stabilizer agent         (source of bismuth ions); and     -   sodium hydroxide to obtain the desired pH.

In order to provide a substantially homogeneous solution and coating, a stir bar is immersed in the aqueous solution and subjected to a rotating field by a magnetic stirrer so that the stir bar rotates in the aqueous solution at a spin rate of about 300 rpm. The magnetic stirrer operates continuously during the preparation of the alkaline solution and during the depositing process.

A first steel substrate is cleaned prior the deposition step. The steel substrate is manually ground on 1200 and 4000 SiC paper sequentially (it may also be possible to use 500 and 1200 MESH SiC paper sequentially), degreased with acetone, rinsed in distilled water and dried. The steel substrate is then activated by a pickling treatment which consists of activating the substrate by immersing it in a 30% acidic solution of HCl (also known as “pickle liquor”) during about 3 minutes in order to remove possible impurities at the exposed surface of the sample. The steel substrate is then rinsed in distilled water to provide a prepared sample ready for coating deposition.

When the pickling treatment is almost over, the coating solution is prepared by adding a reducing agent consisting of sodium borohydride to the alkaline solution with the alkaline solution at a temperature of 95° C. The concentration of reducing agent in the coating solution is about 0.6 g/L.

Immediately after adding the reducing agent to the alkaline solution to form the coating solution, the first prepared sample is immersed in the coating solution during about 1 hour to deposit a nickel-boron coating on the exposed surface of the sample.

At the end of the deposition step, the first sample is removed from the coating solution and rinsed with distilled water and dried in air. Alternatively, it may be dried inert gas, such as nitrogen.

The nickel-boron coating has a chemical composition of about 90.7% wt of nickel, about 6.4% wt of boron and about 3% wt of bismuth. In order to determine the chemical composition of a deposited coating, one method is to dissolve a portion of the sample in aqua regia. Aqua regia is a mixture of ⅓ (molar ratio) of nitric acid and ⅔ (molar ratio) of hydrochloric acid. Once the portion of the sample is dissolved, the resulting solution is analysed by ICP (Inductively coupled plasma optical emission spectrometry). Alternatively, the chemistry is measured by glow-discharge optical emission spectroscopy.

The nickel-boron deposited coating has a thickness of about 15.1 μm. Table 1 below shows comparative data between the nickel-boron deposited coating from a coating solution comprising bismuth ions (hereinafter NiB-Bi) and a nickel-boron deposited coating having a thickness of about 15.42 μm obtained from a comparable coating solution comprising lead ions (hereinafter NiB-Pb) instead of bismuth ions:

TABLE 1 NiB—Bi NiB—Pb Cross section hardness (MPa, hv₁₀₀) 865.21 859.80 Roughness Ra (μm) 0.26 0.20 Roughness Rp (μm) 0.60 0.49 Critical load for first damage Lc (N) 16 30 Friction coefficient μ 0.37 0.41 Wear tracks width (μm) 351 323 Specific wear rate Ws (μm²/N) 3.23 2.53

The NiB-Bi nickel-boron coating of this example has properties comparable with a nickel-boron coating deposition with a coating solution using a lead stabilizer. Furthermore, first tests of corrosion resistance, using a measure of current density with respect of potential, seem to show that the nickel-boron-bismuth coating has better corrosion resistance than the nickel-boron-lead coating.

A coating from a coating solution comprising tin ions Ni-B-Sn (hereinafter NiB-Sn) was deposited on a second steel substrate which had been prepared to provide a sample ready for coating deposition in the same way as that described for the first steel substrate above. The second steel substrate, following its preparation was immersed during about 1 h in a coating bath having the following composition:

TABLE 2 Nickel chloride 24 g/l Sodium hydroxide 39 g/l Ethylenediamine 60 ml/l Tin chloride 0.10 g/l Sodium borohydride 0.602 g/l Bath temperature 95° C. Bath pH 12.5

The nickel-boron deposited coating had a thickness of about 18.3 μm.

Table 3 below shows comparative data between the NiB-Sn coating and the NiB-Pb coating having a thickness of about 15.42 μm shown in the previous comparative example.

TABLE 3 NiB—Sn NiB—Pb Cross section hardness (MPa, hv₁₀₀) 794 859.80 Roughness Ra (μm) 0.62 0.20 Roughness Rp (μm) 2.90 0.49 Critical load for first damage Lc (N) 30 30 Friction coefficient μ 0.33 0.41 Wear tracks width (μm) 323 Specific wear rate Ws (μm²/N) 0.11 2.53

Table 4 below shows information on corrosion resistance. Each sample was submitted to salt fog spray test using a neutral salt spray in accordance with ASTM B117. The amount of corrosion of the surface was quantified by image analysis, software Image J, using a SEM, with respect of the exposed surface. The samples are illustrated in FIG. 1. The “time to decomposition” and “palladium test” values are determined using the tests below:

Time to decomposition test: a plating bath free of any samples for deposition, is submitted to plating conditions, notably to a plating bath temperature of 95° C. The “time to decomposition” represents the time before the plating bath starts to react spontaneously (i.e. starts forming nickel powder). Palladium test: palladium chloride PdCl₂ having a concentration of about 650 mg/l is added dropwise over about 25 s to the plating bath once the plating bath reached the plating condition and a plating bath temperature of 95±1° C. Sufficient magnetic stirring (about 400 rpm) was used to dissipate any concentration gradient due to the addition of PdCl2. The amount of palladium chloride added to the plating bath is 0.5 ml per 100 ml of plating bath solution. The time required for the plating bath solution to be decomposed was recorded. The end point, i.e. the onset of bath decomposition, was when the solution became opaque. The values of “time to decomposition” and “palladium test” of table 4 below are the mean values of 5 tests.

TABLE 4 NiB-lowSn NiB-midSn NiB-highSn NiB—Pb NiB NiB—Bi (see Note 1) (see Note 2) (see Note 3) Time to decomposition (s) 21500 19800 15000 21600 25200 27000 Palladium test (s) 204 190 189 178 211 247 Thickness (μm) 16.02 10.1 15.0 14.5 18.3 20.8 Ra (μm) 0.32 0.22 0.26 0.93 0.62 0.39 Hardness hk₅₀ 854 704 839 668 842 867 Lc (N) 25 23 16 28 30 18 Friction coefficient 0.45 0.48 0.45 0.42 0.35 0.51 Specific wear rate Ws 0.63 0.81 0.67 0.81 0.11 1.29 (μm²/N) Duration of salt fog spray 240 240 240 240 240 240 and 504 test (hours) Corroded surface (%) 21.1 3.9 32.6 66.6 17.6 6.4 (240 h) and 9.3 (504 h) Note 1: deposition with concentrations of tin chloride of 0.05 g/L. Note 2: deposition with concentrations of tin chloride of 0.10 g/L. Note 3: deposition with concentrations of tin chloride of 0.20 g/L. 

1-14. (canceled)
 15. A method of depositing a nickel-boron coating comprising at least 75% wt nickel, wherein the method comprises contacting the substrate with a coating solution having a pH of at least 11 comprising: a source of nickel ions; a reducing agent comprising a source of boron ions; a complexing agent; and a stabilizer agent comprising a source of ions selected from bismuth ions, tin ions, tellurium ions, selenium ions, indium ions and gallium ions.
 16. The method of claim 15, wherein the substrate comprises steel.
 17. The method of claim 15, wherein the deposition speed of the nickel-boron coating on the substrate is at least 10 μm/h.
 18. The method of claim 15, wherein the coating solution is lead-free and thallium-free.
 19. The method of claim 15, wherein the coating solution is phosphorus-free.
 20. The method of claim 15, wherein the coating solution has one or more of the following features (a) through (d): (a) a concentration of the complexing agent in the range 50 to 70 g/l; (b) a concentration of the reducing agent in the range 0.25 to 1 g/l; (c) a concentration of the nickel ions from the source of nickel ions in the range 20 to 30 g/l; (d) a concentration of the stabilizer agent in the range 10 to 25 mg/l.
 21. The method of claim 15, comprising preparing the coating solution by: providing an alkaline solution comprising the source of nickel ions, the complexing agent and the stabilizer; adding the reducing agent to the alkaline solution to form the coating solution.
 22. The method of claim 15, further comprising heat treating the nickel-boron coating after its deposition.
 23. The method of claim 22, wherein the heat treating comprises a heat treatment which lasts at least 30 minutes at a temperature of at least the crystallisation temperature of the nickel-boron coating.
 24. The method of claim 22, wherein the heat treating is carried out at a temperature of at least 280° C. and less than 500° C.
 25. The method of claim 15, wherein the nickel-boron coating comprises at least 85% wt of nickel.
 26. The method of claim 15, wherein the nickel-boron coating comprises (in % wt): at least 85% wt of nickel; boron in the range 1.0 to 10% wt; and a material selected from bismuth, tin, tellurium, selenium, indium, gallium and combinations of two or more thereof, in the range 0.1-5% wt.
 27. The method of claim 15, wherein the nickel-boron coating is free of at least one of lead, thallium, and phosphorus.
 28. The method of claim 15, wherein the nickel-boron coating is free of lead, thallium, and phosphorus.
 29. The method of claim 26, wherein the nickel-boron coating has one of the following features (a) through (h): (a) a thickness of at least 2 μm; (b) a wear tracks width of ≤500 μm; (c) a specific wear rate of ≤4 μm²/N; (d) a friction coefficient of ≤0.35; (e) a cross section hardness of ≥700 hv₁₀₀; (f) a roughness Ra of ≤1 μm; (g) a roughness Rp of ≤0.7 μm; (h) a critical load for first damage Lc of at least 15N (scratch test).
 30. The method of claim 15, wherein the reducing agent comprises a borohydride.
 31. The method of claim 15, wherein the complexing agent comprises ethylene diamine.
 32. The method of claim 15, wherein, during the deposition of the nickel-boron coating, the coating solution has a temperature of at least 80° C. and less than 120° C.
 33. A method of depositing a nickel-boron coating on a substrate, wherein the nickel-boron coating comprises at least 85% wt of nickel, boron in the range 1.0% wt to 10% wt, and a material selected from bismuth, tin, tellurium, selenium, indium, gallium and combinations of two or more thereof, in the range 0.1-5% wt, and the method comprising contacting the substrate with a coating solution having a pH of at least 11, the coating solution comprising: a source of nickel ions; a reducing agent comprising a source comprising a borohydride ; a complexing agent comprising ethylene diamine; and a stabilizer agent comprising a source of ions selected from bismuth ions, tin ions, tellurium ions, selenium ions, indium ions and gallium ions; and wherein the coating solution is lead-free, thallium-free and phosphorus free.
 34. The method of claim 33, wherein the coating solution has: a concentration of the complexing agent in the range 50 to 70 g/l; a concentration of the reducing agent in the range 0.25 to 1g/l; a concentration of the nickel ions from the source of nickel ions in the range 20 to 30 g/l; and a concentration of the stabilizer agent in the range 10 to 25mg/l. 